A glow plug for the preheating of a diesel engine and the like is generally equipped with a resistance heater (hereinafter occasionally just referred to as a “heater”). This glow plug is assembled by fixing the resistance heater in a metallic shell, and mounted onto the engine block of a diesel engine by means of a thread cut in an outer cylindrical surface of the metallic shell in such a manner that a front heating end portion of the resistance heater is located within an engine combustion chamber.
The resistance heater includes a resistive heating element (made of a resistive heating wire or an electrically conductive ceramic material) with a positive temperature coefficient of resistance, so that the electrical resistance of the heating element increases with temperature upon energization of the heater. When the energization of the resistance heater is started through the application of a constant power supply voltage, for example, the heating element is low in temperature and in resistance at the initial stage of energization so as to allow the passage of a relatively large electric current through the heater. As the heating element resistance increases with temperature, the energization of the heater is gradually limited. When the temperature distribution of the heating element comes to equilibrium, the heater resistance becomes substantially constant. The heater temperature then reaches saturation.
Under actual usage conditions of the glow plug, however, the heating end portion of the heater in the combustion chamber is cooled due to various external factors, such as fuel injection and swirl, after starting of the engine. When the heating end portion of the heater is cooled, the heater resistance decreases to cause current fluctuations. The minimization of such heater resistance changes is important to attain a stable heating state of the heater, because the amount of heat generated by the heater is in proportion to the square of the electric current applied. In order to limit the heater resistance to within a predetermined range, it is conceivable to employ a control process in which heater energization power is adjusted according to a difference between a current heater resistance value and a target heater resistance value. (Hereinafter, this control process is referred to as a “resistance control process”.) The stabilization of the heating state of the heater by keeping the heater resistance within a predetermined range has great significance for effective engine startability improvement and emission reduction.
In the resistance control process, the accuracy of measuring the heater resistance is an important parameter to obtain an improvement in control stability. The temperature of the front end portion of the resistance heater in the engine combustion chamber is readily changed due to various external factors including fuel injection and swirl as mentioned above. Although the heater resistance varies in response to such temperature changes, the heater resistance has to be measured accurately. There is a certain time lag until the cooling of a surface of the heater becomes reflected through the temperature distribution of the heating element within the heater. If this time lag is large, an instability phenomenon such as overshooting, undershooting or hunting of the heater resistance is likely to occur even though the heater resistance should be kept constant.
It is further conceivable to employ a mounting method by which the glow plug is mounted in such a manner that a rear end portion of the resistive heating element of the resistance heater is hidden in a mounting hole of the engine block. In this case, there arises a large difference in the influence of the cooling delay between the portion of the resistive heating element hidden in the mounting hole and the portion of the resistive heating element located in the combustion chamber without being hidden in the mounting hole. This can result in the occurrence of the above instability phenomenon in the resistance control process.
It has been recently desired that the glow plug have the capability of reaching a saturation temperature in a minimal time, called quick heating performance, for engine startability improvement. For example, Japanese Laid-Open Patent Publication No. 59-60125 discloses a glow plug having a heating coil and a control coil made of a material having a larger positive temperature coefficient of resistance than that of the heating coil and connected in series with the heating coil within the sheath tube, so as to increase its quick heating performance and to prevent excessive increases in the coil temperature. This disclosed type of glow plug is generally mounted with the front-end-side heating coil protruding in the combustion chamber and the rear-end-side control coil being located in the plug hole. The control coil is low in temperature and in electrical resistance at the initial stage of energization, so that the heating coil receives a relatively large electric current to cause a rapid rise in temperature. As the heating coil temperature rises, the control coil becomes heated by such a temperature rise to increase in electrical resistance and thereby limit the passage of electric current through the heating coil. Accordingly, the heater attains a temperature-rise characteristic in which the temperature of the heating coil rises rapidly in the initial stage of energization, and then, reaches saturation under the energization current limiting action of the control coil. In the case of applying the resistance control process to the above type of glow plug, the control coil having a large temperature coefficient of resistance shows a large resistance change in response to heater cooling. However, the resistance control of the control coil in the plug hole follows on temperature changes of the heating coil in the combustion chamber. This can results in a problem that defective conditions are particularly likely to occur due to the cooling delay of the control coil.